Cardiolipin is a unique phospholipid of the mitochondrial inner membrane. Its peroxidation correlates with release of cytochrome c and induction of apoptosis. The phthalocyanine photosensitizer Pc 4 binds preferentially to the mitochondria and endoplasmic reticulum. Earlier Förster resonance energy transfer studies showed colocalization of Pc 4 and cardiolipin, which suggests cardiolipin as a target of photodynamic therapy (PDT) with Pc 4. Using liposomes as membrane models, we find that Pc 4 binds to cardiolipin-containing liposomes similarly to those that do not contain cardiolipin. Pc 4 binding is also studied in MCF-7c3 cells and those whose cardiolipin content was reduced by treatment with palmitate. Decreased levels of cardiolipin are quantified by thin-layer chromatography. The similar level of binding of Pc 4 to cells, irrespective of palmitate treatment, supports the lack of specificity of Pc 4 binding. Thus, factors other than cardiolipin are likely responsible for the preferential localization of Pc 4 in mitochondria. Nonetheless, cardiolipin within liposomes is readily oxidized by Pc 4 and light, yielding apparently mono- and dihydroperoxidized cardiolipin. If similar products result from exposure of cells to Pc 4-PDT, they could be part of the early events leading to apoptosis following Pc 4-PDT.